"Theme" photo contests are a great idea!
But I suggest care in selecting the topic. For example, a "track photos" theme may be a bit restrictive. I have no desire to track my car, nor do I have access to a track. A more general heading, such as "action shots" would allow the track enthusiasts - and the rest of us to submit photos for that month's contest.
Now if you ever have a contest for "best pic of a car at a rocky beach scene with an iceberg floating majestically in the background", I'll probably score highly there!
Allons-Y!